The following description of background art may include insights, discoveries, understandings or disclosures, or associations together with disclosures not known to the relevant art prior to the present invention but provided by the invention. Some of such contributions of the invention may be specifically pointed out below, whereas other such contributions of the invention will be apparent from their context.
In paper and pulp manufacturing the purpose is to obtain end product having a good and uniform quality. One way of ensuring the quality is to perform measurements during the manufacturing process. One of the most common and important measurements in the pulp manufacturing is the measurement of the pulp lignin content. The lignin content of a suspension such as pulp is usually denoted with a kappa number. In standard SCAN-C 1:77, which is known in the field of pulp manufacturing, the kappa number is defined as the amount of potassium permanganate solution with a concentration of 20 mmol/l in millilitres which one gram of dry pulp consumes in the conditions defined in the standard.
The lignin content can be measured in laboratory environment with known methods. However, laboratory measurements are not suitable in manufacturing environments where results must be obtained quickly in the different process stages to enable control of the manufacturing process based on the measurements.
Lignin content of suspensions may be measured with online kappa analysers by using optical measurements. These measurements provide results which may be used in process control. Typically the measurements are performed using pulp consistency sweep and two separate optical wavelengths in separate measurement chambers. The use of two wavelengths requires the use of two separate measurement apparatuses, circulating the pulp in the measurement chambers and the use of pressure to remove air bubbles. The measurement system is easily choked and is complicated.